It was implemented and we had the functionality for a while but there were issues and the functionality was removed. I forget the exact specifics, it was years ago, maybe a little issue tracker archaeology can uncover something.
Rsync does one-direction two pass syncing, where it first transfers then goes through everything once more and resets the timestamps of directories etc. We did the same, sort of.
But we also expect changes from the user in the other direction, which rsync does not. If we have a lot of changes to do in a directory that can take a while (think lots of large files that need to sync; hours potentially). All the while we are doing that we’ll create temp files and stuff, continuously touching the directory modtime. The user may also be doing stuff in the same directory at the same time. What’s the correct modtime on the directory when our sync is done? It depends, becomes generally ill defined, and 
. Also I think the rules for when a directory modtime changes differ between operating systems and file systems.
I suspect I just felt it was more work than it was worth given how few people really care about those timestamps.